The present invention relates to the mixing of two or more SECAM color-T.V. signals to produce a resultant combined SECAM color-T.V. signal which is a linear combination of the original signals.
A process for the mixing of SECAM color-T.V. signals is disclosed in "Fernsehtechnik," Schoenfelder, H., Part 2, Justus von Liebig Verlag, Darmstadt, pp. 16/1B, 16/1 and 16/2. This process recognizes that additive mixing of frequency-modulated signals does not yield a linear combination of the signals being mixed. In a SECAM color-T.V. signal, the luminance component indicates luminance by means of magnitude, but the two color difference signals, transmitted during alternate respective horizontal-line periods, are frequency-modulated. Accordingly, this known mixing process first takes each SECAM color-T.V. signal to be mixed, and separates the frequency-modulated color-information component thereof from the luminance component. The frequency-modulated color-information component is then frequency-demodulated. Because the base (unmodulated) frequency of the carrier signal for the two color-difference signals alternates, having one value for one color-difference signal, and a different value for the other color-difference signal, this frequency-demodulation is performed on a line-by-line basis, to yield the video-frequency color-difference signals in their alternate horizontal line periods. This is done for each of the original SECAM color-T.V. signals to be mixed. Thereafter, the luminance components of the signals are mixed, and separately therefrom the video-frequency color-information signals are mixed; i.e., the mixing of the luminance components of the signals to be mixed and the mixing of the color-information components of the signals to be mixed is performed in parallel, using two mixing channels. Thereafter, the color-information component mix is frequency-modulated and superimposed upon the mixed amplitude-modulated luminance components, to yield a standard SECAM color-T.V. signal which constitutes a linear combination of the original signals, both with respect to luminance and color information. This technique, compared to the mixing techniques used for PAL and NTSC color-T.V. signals, disadvantageously necessitates the use of two mixing channels, one for the luminance information, another for the color information. The two mixing channels hitherto required for mixing SECAM color-T.V. signals inherently increases the cost and bulk of SECAM mixing circuitry, relative to the one-mixing-channel circuitry which can be used for PAL and NTSC color-T.V. signals.